Congenital cataracts, clinically defined as the presence of cataracts at birth, are the result of mutations in genes that are required for proper lens development. The primary objective of this proposal is to identify the gene defective in the human X-linked cataract dental (XLCD) syndrome, a type of congenital cataract associated with microcornea and dental anomalies. Based on phenotype and location on the X chromosome, the Xcat mouse is the best animal model for human XLCD. We have positionally mapped Xcat through an interspecific backcross and identified two markers Dx Was3 I and DxPas18 that show no recombination with Xcat. Yeast artificial chromosome (YAC) and bacterial artificial chromosome (BAC) libraries were screened with these markers to develop a 900kb contig map of the Xcat critical region. The Xcat critical region will be refined through BAC transgenic complementation experiments designed to identify a single BAC that contains the Xcat gene. We will obtain the sequence of this BAC and analyze it for potential transcription units and ex Candidate Xcat genes will be screened for expression in the lens and by Southern zoo blots and database searches for conservation across species. Candidate genes that are conserved and expressed in prenatal lens will be evaluated for expression in Xcat mice. They will also be evaluated for mutations in the Xcat cDNA. The correlation between the mutant Xcat gene and the cataract phenotype will be confirmed by targeted disruption of Xcat in transgenic animals. Finally, the normal mouse Xcat sequence will be used to clone the corresponding normal human gene. After determining the structure of the normal XLCD gene, XLCD patients will be examined to determine if mutations in the Xcat human homolog are present. Identifying the XLCD gene would contribute to our understanding of the molecular events involved in lens development.